package com.yida.common.core.ukey;

import java.math.BigInteger;

/**
 * base class for an elliptic curve
 */
public abstract class ECCurve {
    BigInteger q;
    ECFieldElement a, b;

    public ECCurve(BigInteger q, BigInteger a, BigInteger b) {
        this.q = q;
        this.a = fromBigInteger(a);
        this.b = fromBigInteger(b);
    }

    public abstract ECFieldElement fromBigInteger(BigInteger x);

    public abstract ECPoint decodePoint(byte[] encoded);

    public ECFieldElement getA() {
        return a;
    }

    public ECFieldElement getB() {
        return b;
    }

    /**
     * Elliptic curve over Fp
     */
    public static class Fp extends ECCurve {
        public Fp(BigInteger q, BigInteger a, BigInteger b) {
            super(q, a, b);
        }

        public BigInteger getQ() {
            return q;
        }

        public ECFieldElement fromBigInteger(BigInteger x) {
            return new ECFieldElement.Fp(this.q, x);
        }

        /**
         * decode a point on this curve which has been encoded using point compression (X9.62 s 4.2.1 pg 17) returning
         * the point.
         */
        public ECPoint decodePoint(byte[] encoded) {
            ECPoint p = null;

            switch (encoded[0]) {
                // compressed
                case 0x02:
                case 0x03:
                    int ytilde = encoded[0] & 1;
                    byte[] i = new byte[encoded.length - 1];

                    System.arraycopy(encoded, 1, i, 0, i.length);

                    ECFieldElement x = new ECFieldElement.Fp(this.q, new BigInteger(1, i));
                    ECFieldElement alpha = x.multiply(x.square()).add(x.multiply(a).add(b));
                    ECFieldElement beta = alpha.sqrt();

                    //
                    // if we can't find a sqrt we haven't got a point on the
                    // curve - run!
                    //
                    if (beta == null) {
                        throw new RuntimeException("Invalid point compression");
                    }

                    int bit0 = (beta.toBigInteger().testBit(0) ? 0 : 1);

                    if (bit0 == ytilde) {
                        p = new ECPoint.Fp(this, x, beta);
                    } else {
                        p = new ECPoint.Fp(this, x, new ECFieldElement.Fp(this.q, q.subtract(beta.toBigInteger())));
                    }
                    break;
                case 0x04:
                    byte[] xEnc = new byte[(encoded.length - 1) / 2];
                    byte[] yEnc = new byte[(encoded.length - 1) / 2];

                    System.arraycopy(encoded, 1, xEnc, 0, xEnc.length);
                    System.arraycopy(encoded, xEnc.length + 1, yEnc, 0, yEnc.length);

                    p = new ECPoint.Fp(this, new ECFieldElement.Fp(this.q, new BigInteger(1, xEnc)), new ECFieldElement.Fp(this.q, new BigInteger(1, yEnc)));
                    break;
                default:
                    throw new RuntimeException("Invalid point encoding 0x" + Integer.toString(encoded[0], 16));
            }

            return p;
        }
    }
}
